A diaphragm control apparatus of an interchangeable lens camera includes a photographing lens, a diaphragm control slider which is driven between first and second control ends corresponding to maximum and minimum apertures, a position detector, and a controller which drives the diaphragm control slider toward the first control end at high-speed, stores in a memory, as a temporary origin, a position to which the diaphragm control slider has rebounded after reaching the first control end. Thereafter, the controller compares the current position of the diaphragm control slider with the temporary origin while driving the diaphragm control slider at a low-speed towards the first control end. When the current position is within a predetermined range of the temporary origin, the controller determines the current position as the origin of the diaphragm control slider.